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NITROGEN MANAGEMENT

Spatial Growth and Nitrogen Uptake Variability of Corn at Two Nitrogen Levels

Dep. of Crop and Soil Sci., Cornell Univ., Ithaca, NY 14850

^* Corresponding author (wjc3{at}cornell.edu)

Received for publication July 1, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Crop measurements that predict spatial yield variability and correlate with yields may facilitate development of variable N rate management. We evaluated growth, N concentration, and N uptake of two corn (Zea mays L.) hybrids at two N rates (130 vs. 185 kg ha^-1 at two sites and manure vs. manure + 55 kg N ha^-1 at another site) in 2000 (wet) and 2001 (dry) to help explain corn spatial yield variability that existed at five of six site–years. Biomass, N concentration, and N uptake mostly had no spatial variability at the sixth leaf stage (V6), silking (R1), and physiological maturity (R6). Plant height at V6, which had spatial variability at four site–years, correlated with yields at two sites (0.55 and 0.66) in 2000 and at one site (0.56) in 2001. Plant height at V10, which had spatial variability at all sites, correlated with yields at all sites (0.50–0.64) in 2000 but at only one site (0.69) in 2001. Nitrogen uptake at R6, which correlated with yields at all sites in 2000 (0.25–0.58) and 2001 (0.34–0.65), did little to explain the N response within sites. Stalk NO₃–N concentrations showed no spatial variability, despite spatial variability for residual soil NO₃–N concentrations at all site–years. Residual soil NO₃–N, which had distinct zones of high concentrations in the upper 30 cm in 2001, may provide more useful information than crop measurements for development of variable N rate management.

Abbreviations: dGPS, differentially corrected global positioning system • R1, silking stage • R3, early grain-fill stage • R6, physiological maturity • TDR, time domain reflectometer • Vn, nth leaf stage

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
CORN YIELDS often show spatial variability, which creates a significant challenge for N fertility management because excessive N results in contamination of the environment and inadequate N results in yield and profit losses to the grower (Doerge, 2002). Kranz and Kanwar (1995) estimated that 70% of the NO₃–N that is leached comes from <30% of a corn field. Variable N rate management has the potential to reduce NO₃–N contamination and increase yield and profits by matching N fertilizer rates to the N fertility requirement of corn in specific areas of a field (Dinnes et al., 2002). The challenge for variable N rate management is to identify areas of a field where corn responds to specific N rates with minimum NO₃–N losses to the environment. Detailed crop measurements may aid in the understanding of why corn yields more or less in certain areas of a field (Sadler et al., 2000; Machado et al., 2002).

Plant height and biomass during corn vegetative development showed significant spatial variability in an 8-ha corn field in South Carolina (Sadler et al., 2000). Plant height at the 12th leaf stage (V12, Ritchie et al., 1993) correlated with grain yield in a dry year but not in a wet year in a 2.7-ha field in Texas (Machado et al., 2002). Plant height at 4 and 8 wk after emergence correlated with corn yields at three of five sites in the Corn Belt in the USA (Mallarino et al., 1999). Mallarino et al. (1999) reported that conditions that favored early season corn growth were the most important factors in explaining the spatial yield variability at all five sites.

Machado et al. (2002) reported that biomass of corn at the R1 stage showed spatial variability in a dry year but not in a wet year. Sadler et al. (2000) reported that biomass accumulation at the R1 stage showed less spatial variability compared with biomass of corn during vegetative development. Also, Cox et al. (1993) reported no differences in biomass and N uptake at the R1 stage at N rates of 56, 140, and 225 kg N ha^-1 in a New York study. Muchow (1988), however, reported that biomass and N uptake at the R6 stage were the overriding factors determining corn yields. Durieux et al. (1995) reported that increasing N fertilizer from 123 to 168 kg N ha^-1 resulted in greater N uptake at harvest but did not influence grain yields in a Vermont study. Singer and Cox (1998) reported that grain yield depended more on N uptake after silking rather than total N uptake.

The stalk NO₃–N test, a crop measurement taken at or shortly after the R6 stage (Binford et al., 1990), may help identify areas within a field that respond differently to N fertilizer_. Brouder et al. (2000) reported that it was an excellent test in separating sufficient vs. excessive (1670 mg kg^-1) corn N fertility under Indiana growing conditions. Fox et al. (2001) reported that the stalk NO₃–N test was an excellent test in separating deficient vs. sufficient (250 mg kg^-1) corn N fertility under Pennsylvania growing conditions.

Crop measurements at the V6 stage that accurately predict the spatial variability of corn yields could help in decision-making for in-season variable N rate management. Late-season crop measurements that correlate closely with corn yields may facilitate the development of future variable N rate management strategies for a particular field. The objective of this study was to evaluate whether crop measurements at strategic corn growth stages provide useful information for the development of variable N rate management.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
We formed farmer-researcher partnerships (Karlen et al., 1995) to conduct two field-scale studies on a dairy farm and three field-scale studies on two cash crop farms in central New York in 1999, 2000, and 2001. Each farmer planted two hybrids (Pioneer brand ‘3752’ and ‘37M81’) with 12-row planters at about 80000 kernels ha^-1 using the split-planter technique, which resulted in 12 rows (0.76-m spacing) of one hybrid alternating with 12 rows of the other hybrid. The farmers sidedressed with anhydrous ammonia in alternate rows at two rates at about the V4 stage, which resulted in six rows of two N levels (manure vs. manure + 55 kg ha^-1 of fertilizer N at the dairy sites and 130 vs. 185 kg ha^-1 of fertilizer N at the three cash crop sites) within each hybrid. After constructing yield maps and observing significant spatial variability for corn yields in 1999 (Katsvairo et al., 2003), we selected three sites (Onondaga 1, a dairy site, and Seneca 1 and 3, cash crop sites) to take crop and soil measurements during the 2000 and 2001 growing seasons.

The Onondaga 1 site had predominantly a Honeoye silt loam soil (fine-loamy, mixed active mesic Glossic Hapludalfs) with a Lima silt loam (fine-loamy, mixed superactive mesic Oxyaquic Hapludalfs) in the northwestern region of the field. The Seneca 1 site had predominantly Cazenovia silt loam (fine-loamy, mixed active mesic Glossic Hapludalfs) with some Darien silt loam (fine-loamy, mixed active mesic Aeric Endoqualfs) in the western region of the field. The Seneca 3 site had a Collamer silt loam soil (fine-silty, mixed active mesic Glossaquic Hapludalfs). There was a total of four strips (300 by 36 m) that contained the two hybrid and two N rate treatment combinations at the Onondaga 1 site, five strips (250 by 36 m) at the Seneca 1 site, and two strips (600 by 36 m) at the Seneca 3 site. The number of strips within each field was determined by the field dimensions. Detailed site characteristics and the cultural practices that were used at each site are presented in the companion paper (Katsvairo et al., 2003).

A systematic sampling grid (Wollenhaupt et al., 1994) with a spacing of 50 by 36 m was superimposed over each experimental site in the spring of 1999 to establish sampling points or stations for soil NO₃–N measurements at the V4 stage in 1999, 2000, and 2001. The planting strips ran parallel to the sampling grid at each location. The location coordinates for each station were recorded with a differentially corrected global positioning system (dGPS). We used the dGPS to navigate to the pre-established stations to establish plots for crop and soil measurements in 2000 and 2001. We measured 3 m in length on either side of the station at the V4 stage to create a 6-m plot in length for each hybrid x N subplot (six rows wide) at each station. The interrow between the two hybrids at each station was then instrumented with a time domain reflectometer (TDR, Model 1502B, Tektronix, Beaverton, OR) probe inserted vertically at the 30-cm depth. Measurements were taken on a weekly basis throughout the remainder of the growing seasons in both years. The TDR tracer from the probe was reduced to volumetric water content (Baker and Allmaras, 1990). The volumetric water content measurements presented in this paper are the average of two weekly measurements around the V6, R1, and early grain-fill (R3) growth stages.

Plant densities were estimated in 2000 and 2001 at the V6 stage for the four treatment combinations at each station by counting all the plants in each row along the 6-m length. Two plants from the outside ends of the four inner rows of the 6-m subplot were then harvested for a sample size of 16 plants per treatment combination at the V6 stage. Although the anhydrous knives fertilized alternate rows, which resulted in no application of fertilizer N in the interrows that separated subplots, we limited our sampling to the four inner rows to ensure against border effects. The plant samples were dried at 60°C in a forced-air oven to constant moisture to determine biomass. The samples were then ground in a Wiley Mill, and plant N concentrations were determined by Kjeldahl procedures (AOAC, 1990). Total N uptake was estimated as the product of biomass x whole plant N concentration.

Plant height was taken at the V6 and V10 stage in the four treatment combinations at each station by estimating the height of two plants from the four inner rows of each subplot. At the V10 stage, we carefully observed the variability of plant height within each subplot and across the four subplots at each station. If visual differences in plant height existed within subplots or across the four subplots that could not be attributed to hybrid or N effects, these stations were discontinued from further sampling. We eliminated these plots because the variability within and/or across subplots would confound hybrid and N effects. We were most interested in measuring hybrid and N effects within stations and spatial variability across and not within stations. Consequently, we took crop, soil water, and residual soil NO₃–N measurements at about 75% of the stations that were originally established.

Two plants from the four inner rows of each subplot at each station were harvested at the R1 and R6 stages. Procedures were similar at the R1 stage to those at the V6 stage for biomass and N concentration determination. At the R6 stage, however, we cut a 20-cm section between the 15- and 35-cm lower portion of the stalk for NO₃–N determination. The 20-cm stalk sections were dried and ground, and NO₃–N concentration was determined using a 2 M KCl extract and an autoanalyzer (Apkem Corp, Klackamas, OR). Total N was then determined on the 20-cm section of the stalk and the remaining part of the plant with the same procedures as those used at the R1 stage except that a hammer mill was used before the Wiley Mill in the grinding operation.

The farmers harvested the corn in late October to mid-November with six-row combines equipped with yield monitors (Katsvairo et al., 2003). Immediately after harvest, soil samples were taken at the 30-cm depth at each station from the high and low N treatment in the main plot of one hybrid, 3752. A total of 10 soil cores were taken in each high and low N treatment. Subsamples from each 10-core composite were sealed in plastic bags, placed in a cooler in the field, transferred to cardboard cartons at the end of the day, and dried in a forced-air oven (55°C) to constant moisture. The samples were analyzed colorimetrically with the autoanalyzer.

We used an analysis of variance (ANOVA) model to analyze each site–year comparison using the General Linear Model (GLM) procedures of the SAS statistical software package (SAS Inst., 1999). We included strips as a random variable in the model, but the strips were not significant for most measurements so they were removed from the model. We then analyzed the data at each site–year comparison with stations (main plot) as a random variable, and hybrids (subplot) and N levels (sub-subplots) as fixed variables in a split-split block design. Station, which was used as an indicator of spatial variability, was tested against the error term of station x hybrid + station x N - station x hybrid x N. Hybrid main effects were compared against the station x hybrid error term. A systematic plot layout, however, does not allow for accurate statistical analysis of hybrid main effects so the results were interpreted qualitatively. Nitrogen levels were tested against the station x N level interaction and all two-way interactions were tested against the three-way interaction. Mean separation between N levels was obtained by the t test. All effects were considered significant at P = 0.05. We also used the regression (REG) procedure in SAS to determine linear and quadratic relationships between the yield and measured variables at each station. Quadratic regression equations were not significant for any variable so only the simple correlation coefficients between yield and the measured variables are reported.

Geostatistics were then used to analyze spatial variability and create spatial maps of volumetric water content, total N uptake at the R6 stage, and residual soil NO₃–N concentrations using the geostatistical software package, Arc GIS (ESRI, 2001). Sample variograms were fitted with spherical variogram models (best fit) using the following equation:

where (h) is the spatial structure of the variable, h is the distance between sampling locations, c_o is the nugget component of the variogram, c is the positive variance component, and a is the variogram range. The variogram range is the distance beyond which spatial correlation of the data no longer exists. The nugget value represents unsampled spatial variation or the random component of the variation. The ratio of the nugget (c_o) to sill (c_o + c) indicates the degree of randomness in the spatial variability of the data. Cambardella and Karlen (1999) suggested that a ratio of <0.25 indicates the measured variable is strongly spatially dependent. A ratio of 0.25 to 0.75 indicates moderate spatial dependence, whereas a ratio >0.75 indicates weak spatial dependence.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
All sites received 620 to 700 mm of precipitation from April through August in 2000, but only 250 to 300 mm in 2001 (Katsvairo et al., 2003). All sites had mostly uniform volumetric water contents in the upper 30-cm soil depth at the V6 stage in 2000 (>0.30 m³ m^-3) and 2001 (0.15–0.30 m³ m^-3). The northeastern region at the Onondaga 1 site had wetter soil conditions compared with most other regions at the R1 stage in 2000 and 2001 (Fig. 1a and 1b) . A small portion of the southwestern region of the Seneca 1 site had wetter soil conditions compared with the rest of the field at the R3 stage in 2000 and 2001 (Fig. 1c and d). The southern region of the Seneca 3 site had wetter soil conditions in the southern compared with the northern region at the R1 stage in 2000 and 2001 (Fig. 1e and f). All three sites thus showed some spatial variability for volumetric water content in the upper 30-cm depth in a dry and wet growing season. The nugget/sill ratios of the variograms were relatively high (Table 1), however, which indicate that a relatively large portion of the spatial variability existed at distances shorter than the sampling intervals. Most soil water measurements did not correlate with grain yields, which had significant spatial yield variability at all site–year comparisons except at the Onondaga 1 site in 2000 (Fig. 2) .

View larger version (53K): [in this window] [in a new window]   Fig. 1. Kriged interpolations, using ArcGIS, of volumetric water content in the upper 30-cm soil depth in 2000 and 2001 at the R1 stage at Onondaga 1 (a, b), at the R3 stage at Seneca 1 (c, d), and at the R1 stage at Seneca 3 (e, f) sites.

View larger version (57K): [in this window] [in a new window]   Fig. 2. Corn yields in 2000 and 2001 using ArcView at Onondaga 1 (a, b), Seneca 1 (c, d), and Seneca 3 (e, f) sites.

Onondaga 1
Nitrogen rate did not have a significant effect on N uptake at the R6 stage at the Onondaga 1 site in 2000 but there was a station x N rate interaction (Table 2). Corn in the high N treatment, which yielded 0.4 Mg ha^-1 greater but economically the same as the low N treatment (Katsvairo et al., 2003), took up between 200 and 250 kg ha^-1 throughout the Onondaga 1 site, except for small sections in the southwestern and northwestern regions (Fig. 3) . Corn in the low N treatment took up between 200 and 250 kg ha^-1 in most of the southern and western regions but only 150 to 200 kg ha^-1 in the north central and northeastern regions (Fig. 3). Corn yields, however, showed no spatial variability or station x N rate interactions, despite differences in N uptake between N rates in the northeastern and southwestern regions. Stalk NO₃–N concentrations exceeded 1670 mg kg^-1 in both treatments, which indicate excessive N fertility (Brouder et al., 2000), probably because of relatively low yields in the excessively wet growing season (Table 5). Evidently, the low N treatment resulted in excessive N fertility even in the north central and northeastern regions of the field where it took up 50 kg ha^-1 less N compared with the high N treatment.

View larger version (56K): [in this window] [in a new window]   Fig. 3. Kriged interpolations, using ArcGIS, of N uptake at physiological maturity in the low and high N treatments at the Onondaga 1 site in 2000 (a, b) and the Seneca 3 site in 2001 (c, d).

Residual soil NO₃–N concentrations in the upper 30-cm soil depth had significant station and N rate effects at the Onondaga 1 site in 2000 and 2001 (Table 5). The low N treatment had residual soil NO₃–N concentrations in the 15 to 30 mg kg^-1 range throughout the Onondaga 1 site in both years except for the northwestern region in 2001 when concentrations ranged from 30 to 45 mg kg^-1 (Fig. 4) . The high N treatment had residual soil NO₃–N concentrations in the 30 to 45 mg kg^-1 range in the northern 33% of the field in 2000 and throughout the field in 2001 except for the northeast region (Fig. 4). The Onondaga 1 compared with the Seneca 1 and Seneca 3 sites had greater nugget/sill ratios for the variograms of residual soil NO₃–N concentrations (Table 1), which indicate more spatial variability at distances shorter than the sampling intervals. The greater small-scale variability in soil N at the dairy site, probably associated with decades of disposing animal waste, makes it more difficult to develop variable N rate management at this site, based on N uptake patterns.

View larger version (74K): [in this window] [in a new window]   Fig. 4. Kriged interpolations, using ArcGIS, of residual soil NO3–N concentrations in the upper 30-cm soil depth in the low and high N treatments at the Onondaga 1 site in 2000 (a, b) and 2001 (c, d).

Corn in the high N treatment had greater N uptake and yield in 2001, but again there were no station x N rate interactions for either measurement despite significant spatial yield variability. Stalk NO₃–N concentrations averaged 155 mg kg^-1 in the low and 372 mg kg^-1 in the high N treatment, which indicates that N fertility limited corn yields in the low N treatment in 2001. Nevertheless, the yield increase at the high vs. the low N rate in 2001 was only 0.3 Mg ha^-1, which was statistically significant but only marginally economic (Katsvairo et al., 2003). The low N treatment had residual soil NO₃–N concentrations below 15 mg kg^-1 in 2001 in most of the western region of the Seneca 1 site and from 15 to 30 mg kg^-1 in most of the south central and eastern regions (Fig. 5) . The high N treatment had concentrations in the 15 to 30 mg kg^-1 range in the western region but from 30 to 45 mg kg^-1 or even above 45 mg kg^-1 in the central and eastern regions (Fig. 5). The N uptake measurements helped explain the overall yield response to the higher N rate in both years at this site. The N uptake measurements, however, did little to explain the spatial yield variability that existed in both years and the distinct residual soil NO₃–N patterns in 2001.

View larger version (44K): [in this window] [in a new window]   Fig. 5. Kriged interpolations, using ArcGIS, of residual soil NO3–N concentrations in the upper 30-cm soil depth in the low and high N treatments taken immediately after harvest in 2001 at the Seneca 1 (a, b) and Seneca 3 (c, d) sites.

Nitrogen rate did not affect N uptake or grain yield but there were station x N rate interactions for both measurements at the Seneca 3 site in 2001 (Table 3). In the high N treatment, N uptake was mostly 200 to 250 kg ha^-1 in the northern region and 100 to 150 in 66% and 150 to 200 kg ha^-1 in 33% of the southern region (Fig. 3). In the low N treatment, N uptake was 100 to 150 and 150 to 200 kg ha^-1 in roughly equal sections of the southern region and 150 to 200 kg ha^-1 in most of the northern region except for 200 to 250 kg ha^-1 in the very northern region (Fig. 3). Corn, however, responded to the higher N rate in about 30% of the southern region where yields and N uptake were the least, and did not respond in the northwestern region where yields and N uptake were the greatest (Katsvairo et al., 2003). The N uptake patterns helped explain the lack of N response at this site but did not identify the potential N management zone in the southern region where corn responded to the higher N rate in 2001.

Station and N rate affected residual soil NO₃–N concentrations in the upper 30-cm soil depth at the Seneca 3 site in 2001. Residual soil NO₃–N concentrations were <15 mg kg^-1 in most of the field in the low N treatment and in the southern 25% of the field in the high N treatment (Fig. 5). Residual soil NO₃–N concentrations, however, ranged from 15 to 30 mg kg^-1 (approximately 23 mg kg^-1 avg.) in the middle two-thirds of the Seneca 3 site in the high N treatment (Fig. 5). This zone of relatively high residual soil NO₃–N concentrations in the high N treatment overlapped with zones where corn took up between 150 and 250 kg N ha^-1. The N uptake data did little to explain the zone of relatively high residual soil NO₃–N concentrations in the high N treatment.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Plant height measurements showed significant spatial variability but did not consistently correlate with corn yields in a dry year. Nevertheless, more research should be conducted on the use of plant height measurements, especially as it relates to timing, because of its simplicity in use and its consistent correlation with yield in a wet year. Nitrogen uptake at harvest, which is a very labor-intensive measurement, did not explain the N response within sites so it is questionable whether this measurement is worth the effort. Residual soil NO₃–N measurements, which are also very labor intensive, showed significant spatial variability at all sites in a wet and dry year. Furthermore, residual soil NO₃–N measurements showed distinct zones of high NO₃–N concentrations in the upper 30-cm depth in the high N treatment in the dry year. Residual soil NO₃–N compared with crop measurements may provide more useful information, especially from the environmental perspective, in the development of variable N rate management strategies.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

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